Re: [dpdk-dev] [PATCH v3] lib/timer: relax barrier for status update

[Phil Yang <Phil.Yang@arm.com>]

> -----Original Message-----
> From: Thomas Monjalon <thomas@monjalon.net>
> Sent: Sunday, April 26, 2020 1:18 AM
> To: Phil Yang <Phil.Yang@arm.com>
> Cc: erik.g.carrillo@intel.com; rsanford@akamai.com; dev@dpdk.org;
> david.marchand@redhat.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; Gavin Hu <Gavin.Hu@arm.com>; nd
> <nd@arm.com>
> Subject: Re: [dpdk-dev] [PATCH v3] lib/timer: relax barrier for status update
> 
> 24/04/2020 09:24, Phil Yang:
> > Volatile has no ordering semantics. The rte_timer structure defines
> > timer status as a volatile variable and uses the rte_r/wmb barrier
> > to guarantee inter-thread visibility.
> >
> > This patch optimized the volatile operation with c11 atomic operations
> > and one-way barrier to save the performance penalty. According to the
> > timer_perf_autotest benchmarking results, this patch can uplift 10%~16%
> > timer appending performance, 3%~20% timer resetting performance and
> 45%
> > timer callbacks scheduling performance on aarch64 and no loss in
> > performance for x86.
> >
> > Suggested-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> > Signed-off-by: Phil Yang <phil.yang@arm.com>
> > Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> > Acked-by: Erik Gabriel Carrillo <erik.g.carrillo@intel.com>
> [...]
> > --- a/lib/librte_timer/rte_timer.h
> > +++ b/lib/librte_timer/rte_timer.h
> > @@ -101,7 +101,7 @@ struct rte_timer
> > -	volatile union rte_timer_status status; /**< Status of timer. */
> > +	union rte_timer_status status; /**< Status of timer. */
> 
> Unfortunately, I cannot merge this patch because it breaks the ABI:
> 
>   [C]'function void rte_timer_init(rte_timer*)' at rte_timer.c:214:1 has some
> indirect sub-type changes:
>     parameter 1 of type 'rte_timer*' has sub-type changes:
>       in pointed to type 'struct rte_timer' at rte_timer.h:100:1:
>         type size hasn't changed
>         1 data member changes (2 filtered):
>          type of 'volatile rte_timer_status rte_timer::status' changed:
>            entity changed from 'volatile rte_timer_status' to 'union
> rte_timer_status' at rte_timer.h:67:1
>            type size hasn't changed
> 

I think we can revert it to the original definition of rte_timer and keep the union rte_timer_status volatile-qualified. 
Because with or without this 'volatile' qualify, it generates the same code on aarch64 and x86.
So it seems acceptable to ignore it to make the ABI compatible?

Thank,
Phil